Project Summary/Abstract We face a public health crisis due to antibiotic resistance, making it imperative to understand how bacteria adapt to antibiotics. The bacterial DNA damage response (SOS response), is a genetic circuit that coordinates the expression of genes linked to the acquisition of resistance. Our data point to a circuit mechanism which enables an extreme separation of error-free and error-prone repair activities at high doses of DNA damage. We believe understanding this mechanism is important, as it may promote the acquisition of antibiotic resistance and thus reveal a novel target for therapy. In this proposal we explore the mechanisms and consequences of temporal gene expression for SOS-induced mutagenesis through the following specific aims: Aim 1. What factors dictate the extent and timing of promoter activity for SOS genes? LexA affinity, SOS gene promoter structure, and the type of DNA damage may all influence the extent and timing of promoter activity within the SOS gene network. We propose to pair biochemistry with a synthetic biology approach to understand how each of these individual factors independently impacts timing in the circuit in order to elucidate the underlying mechanisms responsible for temporal control of promoter activity. Aim 2. What is the mechanism of dose-dependent timing of promoter activities? The lexA promoter, itself, contains binding sites for LexA, placing the SOS-circuit under negative autoregulation. Our data suggest that functional disruption of autoregulation at high doses of DNA damage is critical to achieve the extreme timing differences we observe. We propose to engineer bacterial strains with altered autoregulation of the SOS response to understand how timing of gene expression is achieved. Aim 3. Is the temporal ordering of SOS promoter activities functionally important? Enzymes involved in error-free repair and those involved in error-prone repair may compete for the same damaged DNA substrates. Appropriate timing of these activities may be critical to promote resistance. To test this idea we will engineer bacterial strains with altered timing of these two activities and assess for effects on survival, fitness, and mutational phenomena when exposed to genotoxic antibiotic stress. These studies will uncover new mechanisms for how bacteria adapt to stress and control the timing of gene expression. The information will predict the behavior of other genetic circuits and will inform new approaches in antibiotic drug discovery that aim to suppress mutagenesis in order to prevent the acquisition of antibiotic resistance mutations. It will also extend the PI, who is well versed in biochemical studies, into new areas involving synthetic biology, bacterial genetics, and whole genome sequencing. The combination of a dedicated mentoring team, rigorous plans for career development, and opportunities for integration into a vibrant research community at the University of Pennsylvania will position the PI to become a leading independent researcher dedicated to addressing the problem of antibiotic resistance.